BAGHDAD — Iraq was thrown into a fresh political crisis on Wednesday after the vice president vetoed a newly passed election law, delaying the vote, setting off fresh sectarian wrangling and possibly complicating plans to withdraw American troops.

In a move that caught American officials by surprise, Vice President Tariq al-Hashemi said Wednesday that he had vetoed the new election law the night before; he had threatened a veto but the Americans did not expect him to follow through. Shortly afterward, the chief executive of Iraq's United Nations-supported electoral commission said in an interview for the first time that the elections would have to be delayed.

The veto touched off a political explosion. Iraq's prime minister, Nouri Kamal al-Maliki, condemned it as constitutionally questionable, while President Jalal Talabani, a Kurd, warned that delaying the election risked creating a constitutional vacuum during which the Iraqi government would lose its legitimacy.

Other Kurdish leaders threatened to boycott the elections altogether.

"Parliament could amend this law in a day," Hashemi said. "We have no time to lose." But with Kurdish leaders also objecting to provisions of the law, a much more protracted debate in parliament is likely. Kurdish leaders also want a greater share of parliamentary seats and on Tuesday had threatened to boycott the elections unless their demand was met.

The United States has consistently pushed national elections both as an important step toward reconciling still yawning Iraqi political divisions and as a prerequisite for withdrawing American troops. A postponement could affect the schedule, since American military officials had said they would begin a major drawdown a month or two after elections originally set for Jan. 18.

On Wednesday, however, the top American commander in Iraq, Gen. Ray Odierno, played down the threat, saying that it was "unclear" what the veto meant but that the drawdown could proceed as scheduled despite delays in the election date.

"I think we are set up and are flexible enough between now and the first of May, frankly," Odierno said at a news conference that was apparently originally convened to praise Iraq's progress toward elections. "So I feel very confident that we don't need to make any decisions until the late spring."

The United States has about 120,000 troops in Iraq, a number that President Barack Obama has pledged to reduce to 50,000 by next August, a major logistical undertaking.

Iraq's Presidency Council is made up of both of Iraq's vice presidents, one a Sunni, the other a Shiite, and the president, who is a Kurd. Under Iraq's Constitution, all three must approve any new law passed by parliament. Talabani said he was unhappy with the election law, too, but was ready to sign it to avoid further delays.

Hashemi, the Sunni vice president, said he objected to a clause giving only 5 percent of the seats in parliament to minorities and Iraqi refugees abroad. Iraq has a population of at least 25 million, of whom 2 million — 8 percent — have fled abroad, according to U.N. figures.

Ibrahaim al-Sumydaie, a political analyst here, said he believed that Hashemi's veto was motivated at least in part by political ambitions; he has been expected to announce that he will join an electoral coalition with two Iraqi politicians who have large constituencies among Iraqis who live abroad.

Baha al-Araji, head of parliament's legal committee, also suggested that Hashemi was backing a Baathist agenda, since backers of Saddam Hussein's government oppose the elections.

"That's absolutely not true," said a spokesman for Hashemi, Shakir Kitab. Kitab called the vice president's motivations to include in the vote more of the Iraqis who fled to live abroad "humanitarian and patriotic."

Hamdiya al-Husseini, the head of Iraq's Independent Election Commission, said in a telephone interview that the election would have to be delayed at least one day for every day it took to pass the election law, which has been postponed 11 times since the election commission's original deadline of Oct. 15.

Publicly, American officials expressed concern rather than alarm. "We're disappointed about the developments today concerning the election law," said Philip Frayne, a spokesman for the U.S. Embassy in Baghdad. "We really hope a quick resolution can be found so the elections can proceed in January."